Evaluation and characterization of biochar on the biogeochemical behavior of polycyclic aromatic hydrocarbons in mangrove wetlands

As the inter-tidal regions between land and ocean, mangrove ecosystems have high polycyclic aromatic hydrocarbons (PAHs) content, and the over accumulation of PAHs in mangrove wetland poses a serious ecological risk to the health of plant and living creatures. Comparison to the agricultural sources -biochar, biochar produced from wetland plant has lower O/C (molar ratio), larger N contents, higher stability and more benefits. However, whether the rhizosphere action occurs in biochar- amended sediment and how to influence the biogeochemical behavior of PAH have rarely been reported. In this context, a leaching procedure and pot experiment (60-d) were performed on migration and transformation of PAH at the sediment, and toxicity and their bioavailability in plant affected by the presence of Kandelia obovate-derived biochar in Southeast China. Root exudates amendments significantly increased the cumulative leaching-loss of pyrene by 36-51 % with or without biochar amendment via continuous diffusion and partition process, and biochar amendments decreased the bioavailability of pyrene (16.8-25.8 %) probably due to a faster pyrene sorption on inter-phase transport against desorption. The regression analysis indicated a significant relationship (p < 0.05) between leachate pH and pyrene concentrations. Notably, the bioaccumulation of pyrene on K. obovate parts had significant negative correlation (p < 0.05) to biochar. The activities of four key antioxidizes (phenylalanine ammonia-lyase, dismutases, peroxidases and catalases) were significantly decreased with the application of biochar. Moreover, biochar plays a positive role in cytochrome C release and phosphatidylserine secretion, and a combined biochar-rhizosphere approach can improve the stress tolerance and resistance of K. obovate with an enhanced synergetic effect, which could be a feasible remediation strategy for alleviating the mangrove sediment contaminated by PAH.